1. Field of the Invention
The present invention relates to a zoom lens and an image projection apparatus having the zoom lens which are, for instance, suitable for a liquid crystal projector apparatus that magnifies and throws an image displayed by an image display element, such as a light valve, onto a screen.
2. Related Background Art
Conventionally, a liquid crystal projector has been widely used in a conference, presentation, or the like as an image projection apparatus that throws an image of a personal computer or the like onto a large screen, thereby allowing people to view the image.
There is a demand that an optical system applied to the liquid crystal projector is capable of projecting an image whose brightness is uniform and luminance is high, so it is desired that a pupil on a liquid crystal display element (reduction conjugate) side is located at infinity, that is, the pupil is so-called telecentric.
Conventionally, as a large-aperture and high-resolution projection lens for the liquid crystal projector, various zoom lenses have been proposed which are capable of compensating for various aberrations by arranging five or six lens groups under an appropriate refractive power condition (Japanese Patent Application Laid-Open No. 2004-70306).
Among the zoom lenses, a projection zoom lens for a liquid crystal projector is known which is composed of six lens groups that are a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having positive refractive power, fourth lens group having negative refractive power, a fifth lens group having positive refractive power, and a sixth lens group having positive refractive power that are arranged in this order from an enlargement conjugate side that is a screen side to a reduction conjugate side that is a display image side, with the second lens group, the third lens group, and the fifth lens group being moved at the time of magnification changing (Japanese Patent Application Laid-Open No. 2001-108900).
Also, an optical-correction-type zoom lens is known which is composed of two groups that are a first lens group having negative refractive power and a second lens group having positive refractive power that are arranged in this order from an object side (enlargement conjugate side) to an image side (reduction conjugate side), with magnification changing being performed by moving only the second lens group (U.S. Pat. No. 5,745,303).
Generally, in a liquid crystal projector that uses a liquid crystal panel as an image display element, it is important that a bright projection optical system is used and no shading exists. For instance, when the projection optical system is not image-side telecentric, brightness unevenness occurs and image quality is adversely affected. In addition, in recent years, there has been a demand for miniaturization of a projection lens resulting from miniaturization of the whole of a projector apparatus.
A multi-group optical system where a projection optical system is composed of multiple lens groups and the multiple lens groups are moved at the time of magnification changing is advantageous in terms of aberration compensation, but there is generally a problem in that the construction of the projection optical system becomes complicated and an increase in size is inevitable.
Also, the zoom lens proposed in U.S. Pat. No. 5,745,303 B as a simple lens construction is a zoom lens for a compact camera. Therefore, the reduction-side telecentricity of the zoom lens is not sufficient for a liquid crystal projector. In addition, the F-value of the zoom lens is around 10, so it is not necessarily possible to provide brightness sufficient for the projector.